This invention relates generally to surface mount filters and more particularly to a surface mount dielectric filter which employs a transmission line disposed on a surface of the dielectric filter in order to achieve improved matching and external interconnection.
The reduced size of mobile and portable radio transceivers have placed increased requirements on the filters employed in providing radio frequency (RF) filtering within the transceivers. To enable further size reduction of such filters (which may be used for receiver preselector functions, transmitter harmonic filters, duplexers, and interstage coupling), the coupling of the filter to external circuitry has been achieved by directly connecting one of the plates of an integral coupling capacitor to a mounting substrate, such as has been shown in U.S. Pat. No. 4,673,902 (Takeda, et al.). In some critical applications, however, placing the coupling capacitor plate close to the edge of the filter creates a variability in the value of capacitance due to the proximity of the substrate (which has a dielectric constant greater than free space) and due to the effects of soldering the capacitor plate to the substrate. Furthermore, if the plate of the capacitor is elongated for any significant portion of a wavelength of the frequencies of interest, the plate develops undesirable capacity to ground which adversely affects the coupling to the resonator.